Elastic properties of tissue have generated considerable interest in medicine due to their clinical relevance in identifying, quantifying the severity of, and monitoring the progression of diseases. Tissue elastic properties also serve as biomarkers for cancer, since malignancies are typically more firm by palpation.
In dermatology, for example, the mechanical properties of skin are known to vary between different anatomical regions. The mechanical properties are generally age-dependent and are influenced by different physiological conditions as well as skin disease. One such disease is scleroderma, or systemic sclerosis (SSc), a chronic connective tissue disease often accompanied by a thickening or hardening of the skin as the dermis becomes infiltrated with collagen. Subdermal connective tissue sclerosis leads to dermal tethering and reduced skin mobility. SSc is a multi-system disease also characterized by visceral fibrosis. Accordingly, SSc can cause serious damage to internal organs including the lungs, heart, kidneys, esophagus, and gastrointestinal tract. In fact, cardiac involvement is a common finding in SSc and clinical evidence of myocardial disease may be found in 20-25% of SSc patients. At postmortem examination the heart is affected in up to 80% of patients. Thus, skin disease is both a disabling feature of SSc and a predictor of visceral involvement and increased mortality. Accordingly, an improvement in skin disease correlates with improved survival.
Imaging methods such as confocal microscopy, cutaneous ultrasound, and magnetic resonance imaging (MRI) may be used to analyze tissue for conditions like SSc. Confocal microscopy provides an excellent spatial resolution of about 1 μm and highly efficient visualization of the stratum corneum, but has decreased signal-to-noise ratios (SNRs) when imaging the inner layers of the epidermis. The application of MRI to dermatology has become practical with the advent of specialized surface coils that act as sensitive receivers for signals produced by superficial structures. MRI can provide in-depth resolution on the order of 35 to 70 μm, allowing clear delineation and analysis of the epidermis, but is limited by its high cost and large equipment. Cutaneous ultrasound is a high-frequency ultrasound imaging technique that provides an axial resolution on the order of 30 to 60 μm. Current dermatologic A-scan and B-scan ultrasound units, which operate at frequencies of 15-25 MHz (versus 2-10 MHz for internal medical applications), are capable of distinguishing objects only 50 μm thick while surveying skin to a depth of 1 to 2 cm. Several important uses of cutaneous ultrasound have been investigated, including the measurement of skin thickness, assessment of mass lesions, and the general noninvasive evaluation of skin structure and function. While the epidermis and adnexal structures may generally be seen using dermatological ultrasound, cellular features such as the stratum corneum and basal-cell layers cannot be readily distinguished.
Other methods, such as indentometry, ballistometry, twistometry, and the suction method, analyze tissue mechanical properties to assess tissue health. Measurement of skin mechanical properties has been applied to various medical and cosmetic applications, including the evaluation of laser-based skin resurfacing techniques, the relationship between skin elasticity and wrinkle levels, the role of fibroblast integrins α2 and β1 in maintaining dermal skin properties, and cutaneous alterations in children with growth hormone deficiencies. The mechanical properties of skin have also been used to study the facial skin elasticity of diabetic patients and human skin fatigue.
Ballistometry, which was originally developed for evaluating the density of metallic surfaces, is a biomechanical technique in which cutenoues tone is assessed by analyzing the rebound of a light weight from a tissue surface. Plasticoelastic consequences of diseases such as psoriasis, dermal sclerosis, and aging can be analyzed using ballistometry. Twistometry employs a disk that is glued to the skin and rotated to load a selected level of torque onto underlying skin. The angle of twist of the skin around the disk is then measured to determine skin stiffness.
Commercially available devices, such as the Dermaflex and Cutometer, use the suction method to measure skin displacement caused by a suction force exerted over a defined area of the skin. The Dermaflex device is commercially available from Cortex Technology, Hadsund, Denmark. Cutometer is a registered trademark of Courage+Khazaka Electronic GMBH Corporation of Germany. The Dermaflex device determines skin displacement by measuring the electrical capacitance between the skin surface and an electrode placed on top of the suction device. The Cutometer device measures skin displacement using an optical system. The Dermaflex device has large aperture diameter of 10 mm, while the Cutometer device has a variety of probes with aperture diameters ranging between 1 and 8 mm. It is generally thought that larger aperture diameters measure deeper layers of skin and smaller aperture diameters measure superficial skin layers.
The established method of skin assessment is the Rodnan skin score (RSS), from which the modified Rodnan skin score (MRSS) has more recently been developed. Both RSS and MRSS use semi-quantitative manual skin scoring. For example, using the MRSS method, skin properties such as thickness and stiffness are assessed via palpitation and scored using a 4-degree scale. This is performed for 17 regions of the body, leading to a maximum score of 51. Both RSS and MRSS methods can be subject to observer bias and intraobserver and interobserver variabilities of 12 and 25 percent, respectively. Also problematic is the need for investigator training, varying degrees of examiner experience, and uncertainty about the sensitivity of the scores to change over time.
The above-described methods for assessing the mechanical properties of skin have many limitations that reduce their diagnostic value. For example, their measurements are typically dependent on probe size and application and are generally not indicative of subcutaneous tissue's effects on the skin. Accordingly, assessments of tissue health can vary strongly between patients and observers.
It would therefore be desirable to have a system and method for measuring the dynamic mechanical properties of tissues and producing improved assessments of tissue health that are largely independent of the device used in their obtainment.